The Simcoe Mountains Volcanic Field is a group of lava flows and extinct cinder cones located in the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal district, five ma ...
in south-central Washington state, east of Mount Adams. The Northern and central sections of the Simcoe Mountains are located in Yakama Indian Reservation. Although the volcanic field is located near the Cascade Arc of volcanoes, it is a much older
intraplate In geology, anorogenic magmatism is the formation, intrusion or eruption of magmas not directly connected with orogeny (mountain building). Anorogenic magmatism occurs, for example, at mid-ocean ridges, hotspots and continental rifts. This contrast ...
volcanic field, and not a part of the Cascade Arc.
Highest Points
Most of the volcanic cones are 75 m to 250 m taller than the surrounding land, except for Signal Peak Shield, which is 500 m taller than its surroundings. The highest elevation peaks in the Simcoe Mountains are:
* Jennies Butte (6,410 ft; 1,954 m), a
dacite
Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained ( aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhy ...
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
ic
shield
A shield is a piece of personal armour held in the hand, which may or may not be strapped to the wrist or forearm. Shields are used to intercept specific attacks, whether from close-ranged weaponry or projectiles such as arrows, by means of ...
* Castle Rock (5,656 ft; 1,724 m), a basaltic shield
* Signal Peak Shield (5,100 ft; 1,555 m), a 5.7 million year old
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks includ ...
shield volcano
Geologic History
The Simcoe Mountains Volcanic Field is located in the Yakima Fold Belt in the
Columbia River Basalt Group
The Columbia River Basalt Group is the youngest, smallest and one of the best-preserved continental flood basalt province on Earth, covering over mainly eastern Oregon and Washington, western Idaho, and part of northern Nevada. The basalt gro ...
. The eruptions occurred in three periods during the
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Quaternary; the first episode occurred 4.2–3.2 million years ago. the second was from 2.2–1.2 million years ago, and the most recent episode lasted from 1.0 to 0.6 million years ago. The most recent eruption was a trachybasalt lava flow dated to 631,000 ± 27,000 years ago called the "Trachybasalt of Pretty Swamp".
Some of the volcanic activity in Simcoe Mountains happened at the same time as eruptions of Goat Rocks to the northwest. For the last 350,000 years Simcoe was active, there was also activity in the Mount Adams region, but Mount Adams itself had not yet started forming. Indian Heaven, a volcanic field farther to the west, is also generally much younger than Simcoe, becoming active only in the most recent 200,000 years of activity at Simcoe Mountains Volcanic Field. Mount St. Helens, like Mount Adams, entirely postdates activity at Simcoe Mountains.